Concern for the healthy development of the fetus continues to spawn considerable interest in prenatal behavior, yet relatively little is known about how a fetus behaves and responds to changes within its uterine environment. In spite of technological advances, methodological problems and ethical considerations have limited the study of the human fetus to indirect monitoring and postnatal inference. Therefore, direct observation and experimentation is possible only with nonhuman subjects. Recent technical developments now enable precise manipulation of the intrauterine environment of the rat fetus and observation of fetal behavior in utero. With the pregnant female rat surgically prepared by reversible spinal anesthesia and her uterus externalized into a warm saline bath, the intrauterine environment can be manipulated by intra-amniotic injection, chemosensory stimulation delivered by intraoral infusion, and behavior of the fetus directly observed through the uterine wall, through the amniotic membrane, or within an unrestrained fluid medium. These procedural tools now permit study of the developing mammalian fetus in its environment. This Research Career Development Award application outlines a general program of research to investigate the behavioral biology of the rat fetus. The principal objective of the proposed program is threefold: (a) to describe the normal behavioral development of the unmanipulated rat fetus in utero; (b) to characterize the behavior of the fetus in aversive and appetitive learning situations and to explore the role of fetal learning in subsequent behavioral development; and (c) to investigate the interplay between prenatal behavior and the development of behavioral and morphological anomalies.